For the rice, in a saucepan, heat oil and sautée onion with garlic and spices. Add rice and stir. Add water, reduce heat and cover. Simmer until rice has absorbed all the liquid. Remove from heat. Add black beans, green onions and cilantro. Season to taste.
For the avocado sauce, using a food processor or blender, purée all the ingredients and season with salt and pepper. Add water if necessary until smooth.
Transfer the rice to serving bowls and garnish, to taste. Drizzle with the avocado sauce.